1. Field of the Invention
The present invention relates to a process for producing a fluorinated copolymer and a fluorinated copolymer, and more particularly relates to a process for producing a fluorinated copolymer which enables providing a crosslinking product having excellent mechanical properties even if no inorganic additive is added thereto. Furthermore, the present invention relates to a crosslinkable composition containing the fluorinated copolymer and a sealant obtained by crosslinking the crosslinkable composition.
2. Prior Art
Fluorinated copolymers are polymeric materials having excellent heat and chemical resistances. In particular, fluorinated copolymer elastomers are used as materials for forming sealants such as O-rings, packings oil seals and gaskets and hoses which are required to possess heat and oil resistances.
However, the mixing of inorganic fillers such as carbon black and silica into the fluorinated copolymer elastomer has been inevitable for imparting mechanical properties such as hardness and strength and compression set resistance characteristics that are satisfactory for practical use as sealants to vulcanizates of fluorinated copolymer elastomers.
On the other hand, with respect to the sealants for use in the semiconductor industry, medical materials, food industry and the like among those required to possess heat and chemical resistances, the mixing of various inorganic additives such as an inorganic filler, an acid receptive agent, a vulcanization accelerator and a colorant thereinto is not desirable from the viewpoint that it is not desirable to contaminate products and product materials brought into contact with the sealants.
For coping with this dilemma, a process for producing a fluorinated resin which does not require crosslinking, can be easily molded and is flexible has been proposed as disclosed in Japanese Patent Laid-open Publication No. 58(1983)-206615. The "flexible fluorinated resin" described in this published specification comprises fluorinated elastomeric segments and fluorinated crystalline segments and provides molding with satisfactory mechanical properties even if no inorganic filler is added thereto. However, this fluorinated resin is thermoplastic and therefore has a drawback in that it cannot be employed at temperatures over the melting point of the crystalline segments (155 to 160.degree. C.). Further, with respect to the particular process, the production of the fluorinated resin requires a plurality of burdensome steps such as emulsion polymerization, salting out/water washing, drying, solvent washing, drying, graft (solution) polymerization and solvent separation/drying. Thus, this process can hardly be stated as a desirable one from the viewpoint of cost.
Moreover, fluorinated thermoplastic elastomers having desirable mechanical properties are disclosed in Japanese Patent Laid-open Publication No. 53(1978)-3495 and Japanese Patent Publication No. 61(1986)-49327. However, the compression set resistance thereof which is important in the use as sealants is not on a satisfactory level at all. Further, these fluorinated thermoplastic elastomers are thermoplastic and therefore have a drawback in that the use temperature thereof is restricted by the melting point of the crystalline segments thereof.
On the other hand, for example, Japanese Patent Publication Nos. 2(1990)-36365, 5(1993)-18329 and 1(1989) -56659 and Japanese Patent Laid-open Publication No. 59(1984)-40066 describe that these fluorinated thermoplastic elastomers can be crosslinked by radiation and peroxides to thereby enable enhancing the compression set and crushing resistances although the recyclability of the fluorinated thermoplastic elastomers is sacrificed. However, without exception, the production of fluorinated thermoplastic elastomers as base materials is performed by multi-stage block polymerization, so that a cost increase as compared with the common one-stage polymerization is inevitable.
In Japanese Patent Laid-open Publication No. 7(1995)-11086, the applicant disclosed a fluorinated elastomer composition comprising a copolymer obtained by copolymerizing ethylidene fluoride and chlorotrifluoroethylene and a salicylaldimino copper complex as a crosslinking agent. Although this composition has realized desirable vulcanization rate and vulcanizate properties, it is essential for this composition to contain a metal complex as a crosslinking agent and to contain a divalent metal oxide or hydroxide as an acid receptive agent with the result that there has been a difficulty in the application thereof to the above sealants for use in the semiconductor industry, medical materials, food industry and the like, in which the addition of inorganic additives is not suitable. Further, the thus obtained fluorinated elastomer composition is not satisfactory in compression set, so that the application thereof to sealants, especially O-rings, cannot necessarily be stated as being satisfactory.
With respect to the production of vinylidene fluoride/chlorotrifluoroethylene copolymers, U.S. Pat. No. 2,752,332 includes Examples in which vinylidene fluoride and chlorotrifluoroethylene are charged together in a molar ratio of 20/80 to 30/70 and suspension polymerized at low temperature, for example, -20.degree. C. for 18 hr in the presence of a redox initiator composed of a persulfate salt, acid sodium sulfite and a polyvalent metal salt to thereby obtain copolymers whose component ratio (vinylidene fluoride/chlorotrifluoroethylene ratio) is in the range of 24/76 to 25/75 with a yield of 14 to 27% by weight. Also, the mechanical properties of copolymer whose vinylidene fluoride/chlorotrifluoroethylene ratio is 25/75 among the above copolymers are evaluated in the '332 patent. The elongation at break thereof is extremely low to thereby render the application to sealants difficult.
Further, U.S. Pat. No. 2,770,606 discloses Examples in which vinylidene fluoride and chlorotrifluoroethylene are charged together into a reactor in a molar ratio of 50/50 to 75/25 and suspension polymerized at 25 to 35.degree. C. in the presence of an initiator composed only of a persulfate salt or composed of a persulfate salt, acid sodium sulfite and a polyvalent metal salt to thereby obtain vinylidene fluoride/chlorotrifluoroethylene copolymers. In particular, Example 4 of the '606 patent describes that a rubbery copolymer whose vinylidene fluoride/chlorotrifluoroethylene ratio is 75/25 is obtained at a conversion of 70% by carrying out the reaction at 25 to 35.degree. C. for 21 hr in the presence of potassium persulfate as an initiator. However, as a result of the follow-up test by the inventors, it was found that the polymerization reaction minimally proceeded at reaction temperature of 25 to 35.degree. C. in the absence of a reducing agent. In the '606 patent, it is only described that the obtained various copolymers are available in the formation of coating solutions, and there is no description as to the properties which are important in the use as sealants, for example, physical properties such as glass transition temperature and melting point and mechanical properties of moldings.
Still further, U.S. Pat. No. 2,738,343 includes Comparative Examples in which rubbery fluorinated copolymers whose respective vinylidene fluoride/chlorotrifluoroethylene ratios are 34.4/65.6 and 56.5/43.5 are obtained. However, there is no description as to the elastomeric properties of these copolymers, and the polymerization reaction without exception requires a prolonged period of time, for example, 24 to 168 hr. Therefore, it can hardly be stated that a practical productive process is disclosed.
The inventors have conducted extensive and intensive studies with a view toward resolving these problems. As a result, it has been found that the copolymer obtained by copolymerizing vinylidene fluoride and chlorotrifluoroethylene in the presence of a specified compound of the formula I.sub.n Br.sub.m R has excellent mechanical properties and is advantageous in that the production thereof can be effected without the need to conduct complex operations such as multi-stage block polymerization which must be carried out in the production of the above fluorinated copolymers.
The present invention has been made with a view toward solving the above problems. An object of the present invention is to provide a process for producing a fluorinated copolymer which can impart mechanical properties satisfactory for practical use as sealants even if no inorganic additive is added thereto. Further, other objects of the present invention are to provide a crosslinkable composition enabling crosslinking of this fluorinated copolymer and a crosslinked material obtained from the crosslinkable composition.